Milky Way dwarf galaxies crashed in the past

On a clear night, if you look up at the Southern Hemisphere sky, you’ll see two wispy forms—the Small and Large Magellanic Clouds (SMC and LMC). Less than 200 000 light-years from Earth, both are dwarf galaxies, 2 of the more than 20 known to orbit the Milky Way. Using data from the Gaia space telescope, an international team of researchers led by the University of Michigan’s Sally Oey now reports that the two dwarf galaxies collided 100–200 million years ago, a relatively recent event by astronomical standards.

Since its launch in December 2013, Gaia (illustrated at right) has been acquiring precise positions and velocities for the roughly 1 billion stars closest to Earth. Oey and colleagues considered a sample of 315 field stars—that is, stars not in clusters—in the SMC. Most of those stars occupied the dwarf galaxy’s central bar-like region, but a sizable minority occupied a region called the wing, which is slightly displaced from the bar in the direction of the LMC. After subtracting the overall motion of the SMC, the Oey team found that the rapidly moving “runaway” stars in the wing collectively moved toward the LMC with an average speed of about 64 km/s; according to computer simulations carried out earlier by coauthor Gurtina Besla and colleagues, such motion implies that the SMC and LMC collided not so long ago.

Oey and company’s original motivation for studying the kinematics of the SMC field stars was to determine how the runaways came to flee: Were they ejected from star clusters by supernova explosions or thrown out by gravitational instabilities? The researchers found evidence for both mechanisms, but the data suggest that the faster runaways were the product of instabilities. The discovery of the galactic crash was the icing on the cake. (M. S. Oey et al., Astrophys. J. Lett. 867, L8, 2018 .)